1. Field of the Invention
This invention relates to a fuel cell device, and more particularly to a fuel cell device having an electrolyte replenishing function capable of realizing an easy and effective replenishment of electrolyte.
2. Related Art Statement
FIG. 1 is a perspective view, a part of which is omitted, illustrating an example of the general structure of a molten carbonate type of fuel cell device. A fuel cell device of the type described above comprises a fuel cell stacked body that is structured in such a manner that single fuel cells 4 (to be called "cells" hereinafter) are stacked with separator members interposed therebetween. The cell 4 comprises an oxidant gas side electrode 1, a fuel gas side electrode 2, and an electrolyte layer 3 which is interposed therebetween. A positive side terminal member 6a and a negative side terminal member 6b are mounted on the corresponding positive side end and negative side end portions. The above-described positive side terminal and the negative side terminal members 6a and 6b and the separator member 7 each has a gas impermeability, and form reaction gas flow lines 81a and 81b for supplying an oxidant gas and a fuel gas to the oxidant gas side electrode 1 and the fuel gas side electrode 2, respectively. These members 6a, 6b and the separator member 7 each has an electron conductivity so that they act to electrically connect the cells 4 in a series. The side surfaces of the stacked body 5 are provided with gas manifolds 8a and 8b for distributing and supplying (or discharging) that oxidant gas and the fuel gas to the corresponding reaction gas flow lines 81a and 81b. A gasket 9 is interposed between the stacked body 5 and the gas manifolds 8a and 8b to be abutted against each other. An arrow A in the figure designates the oxidant gas flow, while an arrow B designates the fuel gas flow.
FIG. 2 shows a separator member 7a for replenishing electrolyte to which a replenishing pipe 10 is connected, this replenishing pipe 10 replenishing electrolyte from outside of the fuel cell device into the electrolyte layer 3 which is disclosed, for example, in Japanese Patent Laid-Open No. 61-24159. A fuel cell device in which electrolyte can be replenished from outside thereof can be obtained by using the separator member 7a for replenishing electrolyte as the separator member 7.
An operation of the fuel cell device will now be described.
The electrolyte layer 3 in a molten carbonate type of fuel cell is constituted in such a manner that a substance (such as LiKCO.sub.3) which acts as the electrolyte is retained in a porous structure made of a material (such as LiAlO.sub.2) which is chemically stable and which has electrical insulation. It therefore acts as the electrolyte layer of a fuel cell and also acts as a gas separator layer for preventing mixing of the fuel gas to be supplied to the fuel gas side electrode and the oxidant gas to be supplied to the oxidant gas side electrode. If the quantity of the electrolyte contained in the electrolyte layer 3 becomes lacking for some reason, the internal resistance of the cell increases, causing for the cell characteristics to be deteriorated. If it becomes depleted excessively, the gas separating function becomes insufficient, causing the operation of the fuel cell to become difficult because of the resulting partial mixing of the fuel gas and the oxidant gas.
In an actual molten carbonate type of fuel cell since the electrolyte diminishes from the electrolyte layer as the cell works, the insufficient quantity of the electrolyte causes the life of the fuel cell to be restricted because of the above-described reason. For example, the life test to which a cell is subjected resulted substantially in 10,000 hours, and the same to a stacked battery resulted in substantially 5,000 hours. Therefore, it is critical for lengthening the life of the fuel cell and improving the characteristics of the same to prevent depletion of the electrolyte in the electrolyte layer by some measures.
The effect obtained by replenishing the electrolyte performed in the life test for the cell was, as shown in FIG. 3, confirmed by a group including an inventor of the present invention. That is, in this test, it was confirmed that replenishment of electrolyte was effective since the internal resistance was successively reduced and the electrical characteristics were improved. In this test, a first replenishment of the electrolyte was, as designated by a symbol A in FIG. 3, performed 3,200 hours after the start of operation and the second replenishment was, as designated by a symbol B in FIG. 3, performed 5,500 hours after the same.
However, in a conventional device which is, as shown in FIG. 2, constructed in such a manner that the electrolyte is directly replenished from outside to the electrolyte layer 3 of each cell 4 by means of a replenishing pipe 10 which is provided for each separator member 7a, although an excellent replenishment of the electrolyte can be effected similarly to the test result shown in FIG. 3, the structure of the stacked layer becomes excessively complicated.
Since the conventional fuel cell device is structured as described above, problems arise in that the structure of the stacked body is too complicated, causing the whole stacked body to be made thin, and cost to become great. Furthermore, since electrolyte needs in be replenished to each of the cells, the replenishing work becomes complex.
FIG. 4 is a cross-sectional view illustrating an example of a fuel cell device comprising the conventional cells of a type disclosed in Japanese Patent Laid-Open No. 62-98568.
Referring to this figure, a cell 4 comprises, similar to the device shown in FIG. 1, an oxidant gas side electrode 1, fuel gas side electrode 2, and electrolyte layer 3. The cell 4 is sandwiched by cell frames 11, and collector plates 12 are each disposed between the cell frame 11 and the electrode 1 or 2. A surface where the electrolyte layer 3 and the cell frame 11 are positioned in contact with each other is provided with a wet seal 13. An electrolyte retaining member 14 for retaining excessive electrolyte is accommodated in an electrolyte stoking space 15 disposed in the edge portion 11a of the cell frame 11.
FIG. 5 is a cross-sectional view illustrating another example of a fuel cell device which is similar to that shown in FIG. 4 and which comprises the conventional cells of a type disclosed in Japanese Patent Laid-Open No. 62-98568. An electrolyte replenishing pipe 10 for replenishing, via the wet seal portion 13, electrolyte from outside to the electrolyte layer 3 is provided. The remainder structure is constructed similarly to the conventional device shown in FIG. 4.
As described above, replenishment of electrolyte has received growing interest as means for preventing depletion of electrolyte, and a variety of methods of replenishing the electrolyte and the replenishing structures have been examined.
Referring to FIG. 4, the electrolyte retaining member 14 is a member constituted in such a manner that a surplus electrolyte is retained in a porous body such as zirconia felt. When the electrolyte retained in the electrolyte layer 3 becomes depleted, the surplus electrolyte contained in the electrolyte retaining member 14 is first moved to the electrolyte layer of the wet seal 13, next it is distributed over the entire surface of the electrolyte layer 3 due to the capillary phenomenon so that electrolyte replenishment is performed.
At this time, the force to cause the electrolyte to be moved is the difference in the electrolyte retaining force between the electrolyte layer 3 and the electrolyte retaining member 14, and the electrolyte retaining force between the portion of the electrolyte layer 3 lacking in the electrolyte and the portion of the same which sufficiently retains the electrolyte. The difference in the electrolyte retaining force can be obtained by an arrangement conducted in such a manner that the diameter of the small apertures in the electrolyte layer 3 is made smaller than that of the electrolyte retaining member 14.
In FIG. 5, the electrolyte which has been replenished, through the electrolyte replenishing pipe 10, to the electrolyte layer adjacent the wet seal portion 13 is distributed to the portion lacking electrolyte in the electrolyte layer 3 due to the capillary phenomenon so that electrolyte replenishment is performed.
The above described methods of replenishing electrolyte is based on the following two phenomena:
(1) The distribution of the electrolyte in a plurality of porous bodies in an equilibrium state is defined by the electrolyte retaining force of each porous body.
(2) The movement of the electrolyte is a capillary phenomenon based on the electrolyte retaining force.
The conventional electrolyte replenishing means that is based on the above-described phenomena can be effected in small cells having the effective electrode area of, for example, 300 cm.sup.2. However, the following problems arise when a large and long life fuel cell is intended.
(1) The electrolyte retaining force which defines the distribution of the electrolyte can be generally obtained by arranging the small apertures distribution in the porous body which retains the electrolyte. However, since aperture distribution arrangement is difficult and this distribution is changed due to sintering and conversion of the crystal structure, it is difficult for the electrolyte retaining force to be stably controlled for a long time.
(2) Since the movement of the electrolyte is due to the capillary phenomenon, the moving speed is insufficient. The moving speed of the electrolyte greatly depends upon the composition of the electrolyte, the diameter of the small apertures in the electrolyte layer, and the temperature. For example, it takes substantially 1000 hours for the electrolyte to be moved through an electrolyte layer of 30 cm. Since the time required for an electrolyte to be moved through a predetermined length is proportional to the square of the distance of movement, this raises a problem when a large size fuel cell is intended.
(3) For the above-described two reasons, it is difficult for the effective degree of replenishment of the electrolyte which has been replenished from outside to be quantitatively assayed.
Since the fuel cell device having the conventional electrolyte replenishing function is constructed as described above, it is difficult to quickly, uniformly and effectively replenish the electrolyte and a stable and constant electrolyte replenishment cannot be conducted over a long time when the battery has a large size and long life.